«A Design Guide for Earth Retaining Structures Hugh Brooks Civil & Structural Engineer John P. Nielsen Civil & Geotechnical Engineer Basics of ...»
Batter: The slope of the face of the stem from a vertical plane, usually on the inside (earth) face.
Dowels: Reinforcing steel placed in the footing and bent up into the stem a distance at least equal to the required development length.
Footing (or foundation): That part of the structure below the stem that supports and transmits vertical and horizontal forces into the soil below.
Footing key: A deepened portion of the footing to provide greater sliding resistance.
Grade: The surface of the soil or paving; can refer to either side of the wall.
Heel: That portion of the footing extending behind the wall (under the retained soil).
Horizontal temperature/shrinkage reinforcing: Longitudinal horizontal reinforcing usually placed in both faces of the stem and used primarily to control cracking from shrinkage or temperature changes.
Keyway: A horizontal slot located at the base of the stem and cast into the footing for greater shear resistance.
Principal reinforcing: Reinforcing used to resist bending in the stem.
Retained height: The height of the earth to be retained, generally measured upward from the top of the footing.
Stem: The vertical wall above the foundation.
Surcharge: Any load placed in or on top of the soil, either in front or behind the wall.
Toe: That portion of footing which extends in front of the front face of the stem (away from the retained earth).
Weep holes: Holes provided at the base of the stem for drainage. Weep holes usually have gravel or crushed rock behind the openings to act as a sieve and prevent clogging. Poor drainage of weep holes is the result of weep holes becoming clogged with weeds, thereby increasing the lateral pressure against the wall. Unless properly designed and maintained, weep holes seldom “weep”.
Alternatively, perforated pipe surrounded with gravel and encased within a geotextile can be used to provide drainage of the backfill.
1. That it has an acceptable Factor of Safety with respect to overturning.
2. That it has an acceptable Factor of Safety with respect to sliding.
3. That the allowable soil bearing pressures are not exceeded.
4. That the stresses within the components (stem and footing) are within code allowable limits to adequately resist imposed vertical and lateral loads. It is equally important that it is constructed according to the design.
Design Criteria Checklist Before establishing specific design criteria, the following checklist should be used before starting
What building codes are applicable?
Do I have the correct retained height for all of my wall conditions?
Is there a property line condition I need to know about?
Is there a fence on top of the wall, or does the wall extend above the retained height?
(exposure to wind) How deep must the bottom of the footing be (frost considerations?)?
How will I assure that the backfill will be drained?
Will there be any axial loads on top of the wall? If so, the eccentricity?
What about surcharges behind the wall, such as parking, trucks, etc.
If the wall extends above the higher grade, and is a parking area, is there an impact bumper load?
What is the slope of the backfill? Level?
Is there a water table I need to consider?
Is a seismic design required?
Are there any adjacent footing loads affecting my design?
Should the stem be concrete or masonry, or a combination of the two?
How high is the grade on the toe side, above the top of the footing?
Is there a slab in front of the wall to restrain sliding or provisions to prevent erosion of soil?
Is there lateral restraint at the top of the wall (if so, it’s not truly a cantilevered wall and requires a different design)?
Do I have a soil investigation report or other substantiation for soil properties: active pressure, passive pressure, allowable bearing pressure, sliding coefficient, soil density, and other items I need to consider?
Also consider whether a cantilevered retaining wall is the right solution. If the height of the wall is over about 16 feet, perhaps a tieback wall would be more economical (caution: be sure your client has the right to install tiebacks. If the wall is on a property line, there is obviously a problem). Perhaps a buttressed or counterfort wall would be better for high walls, or using
2. DESIGN PROCEDURE OVERVIEW Page 7 Basics of Retaining Wall Design precast panels, or tilt-up to overcome construction constraints imposed by a restrictive rear property line.
Lastly, determine how many conditions for which you will need a design. Perhaps the same retained height has several different backfill slopes, say, from level to 2:1. Here you need to use a little judgment in determining the number of cases. Usually you don’t design for less than two-foot height increments, unless there are different surcharges or other conditions. To design for one-foot height increments is not only tedious, but doesn’t save that much material cost. On the other hand, if the retained height along the length of a wall varies from, say, four feet to 12 feet, you would not want to specify the 12-foot design throughout. In this case, you would probably design for 12', 10', 8', 6' and 4'. You rarely “design” a wall less than 4 feet high, just use a little judgmentunless there is a steep backfill slope or large surcharges, in which case it should be properly designed.
Establish Design Criteria The following information will be needed before starting your design. The values shown in parentheses are only given to illustrate those values frequently used.
Basic Design Principals for Cantilevered Walls Stability requires that a cantilevered retaining wall resists both overturning and sliding, and material stresses including the allowable soil bearing that must be within acceptable values.
To resist forces tending to overturn the wall (primarily the lateral earth pressure against the back of the wall), the wall must have sufficient weight, including the soil above the footing, such that the resisting moments are greater than the overturning moments. The safety factor for overturning should be at least 1.5 – some codes require more.
2. DESIGN PROCEDURE OVERVIEW Page 8 Basics of Retaining Wall Design To resist sliding, the weight of the wall plus the weight of the soil above the footing plus vertical loads on the wall and any permanent surcharges multiplied by the coefficient of friction between the foundation soil and the bottom of the footing, plus the passive pressure resistance force at the front of the wall, must be sufficient to resist the lateral force pushing on the wall. The recommenced safety factor against sliding is 1.5. If the soil is cohesive, the coefficient of friction is replaced by the adhesive bond (see page 20) of the cohesion between the footing and soil, in psf.
The stem must be designed to resist the bending caused by earth pressures, including the effect of surcharges placed behind the wall, seismic or wind if applicable, impact loads, or axial loads acting eccentrically on the wall. The maximum bending and shear stresses in a cantilevered wall will be at the bottom of the stem. Each of these subjects will be discussed later.
Figure 2.1 is a free-body force diagram illustrating stability forces on a cantilevered wall.
Step-by-Step Design of a Cantilevered Retaining Wall
The design usually follows this order:
1. Establish all design criteria based upon applicable building codes. (See checklist above).
2. Compute all applied loads, soil pressures, seismic, wind, axial, surcharges, impact, or any others.
3. Design the stem. This is usually an iterative procedure. Start at the bottom of the stem where moments and shears are maximum. Then, for economy, check several feet up the stem (such as at the top of the development length of the dowels projecting from the footing) to determine if the bar size can be reduced or alternate bars dropped. Check dowel embedment depth into the footing assuming a 90° bend (hooked bar). The thickness of the stem may
2. DESIGN PROCEDURE OVERVIEW Page 9 Basics of Retaining Wall Design vary, top to bottom. The minimum top thickness for reinforced concrete walls is usually 6inches to properly place the concrete, 8-inches at the bottom.
4. Compute overturning moments, calculated about the front (toe) bottom edge of the footing.
For a trial, assume the footing width, to be about 1/2 to 2/3’s the height of the wall, with 1/3 being at the toe.
5. Compute resisting moments based upon the assumed footing width, calculated about the front edge of the footing.
6. Check sliding. A factor of safety with respect to sliding of 1.5 or more is standard. A key or adjusting the footing depth may be required to achieve an accepted factor of safety with respect to sliding.
7. An overturning factor of safety of at least 1.5 is considered standard of practice.
8. Based upon an acceptable factor of safety against overturning, calculate the eccentricity of the total vertical load. Is it within or outside the middle-third of the footing width?
9. Calculate the soil pressure at the toe and heel. If the eccentricity, e, is B/6 (B = width of footing) it will be outside the middle third of the footing width (not recommended!), and because there cannot be tension between the footing and soil, a triangular pressure distribution will be the result. Consult with the project geotechnical engineer if this condition cannot be avoided, as it will result in a substantially lowered allowable soil bearing pressure.
See Figure 8.4.
10. Design footing for moments and shears. Select reinforcing.
11. Check and review. Have all geotechnical report requirements been met?
12. Place a note on the structural sheets and on the structural calculations indicating that the backfill is to be placed and compacted in accordance with the geotechnical report.
13. Review the construction drawings and specifications for conformance with the design.
Step-By-Step Design of a Restrained Retaining Wall
Similar to the above with some additional steps (italicized):
1. Establish all design criteria based upon applicable building codes. (See checklist above).
2. Compute all applied loads (at-rest earth pressures, seismic, wind, axial, surcharges, impact, or any others. Select “height” to lateral restraint.
3. Select restraint – level and base of stem design assumptions: pinned - pinned; pinned fixed;
or fixed - fixed. Then based on statics determine the reactions at the top and at the base of the wall.
4. If a floor slab is present at the top of the footing, check its adequacy to sustain this lateral sliding force.
5. Design the stem. If the stem is assumed pinned at the base and at the top, the maximum moment will be a positive moment near mid-height—select stem material, design thickness, and reinforcing for that location. Usually the same material (concrete or masonry) and thickness will be used for the full height. Some degree of “fixity” is likely at the top of the wall even with a pinned “design”.
6. Design the footing. If the stem is assumed fixed at the base check the soil pressure (check Items 8 and 9 as above) and design for the moments and shears, and select reinforcing. If the
2. DESIGN PROCEDURE OVERVIEW Page 10 Basics of Retaining Wall Design stem is assumed pinned at the footing interface, try to center the footing under the wall to prevent eccentricity. If there is eccentricity check reinforcing at stem-footing interface to resist that moment because if it exceeds the moment due to eccentricity the soil pressure will not be uniform Check embedment depth into the footing assuming a 90° bend (hooked bar).
7. Check sliding. If a restraining floor slab is not present, a key or adjusting the footing width or depth may be required.
8. Check and review. Have all soil report requirements been met?
9. Review the construction drawing for conformance with your design.
All these topics will be discussed later.